1. Field of the Invention
This invention relates to an image forming apparatus using e.g. an electrophotographic system, and more particularly to an image processing apparatus for heating and melting a toner image formed on a recording medium so that it is fixed on the recording medium.
2. Description of the Related Art
In an image forming apparatus such as a printer or a copier using an electrophotographic system, an electrostatic latent image is formed on the surface of an image carrier of a drum or belt-shaped organic photosensitive material through a known electrophotographic process. Thereafter, the electrostatic latent image is developed using toner to provide a toner image. Subsequently, the toner image is electrographically transferred on a recording medium (paper sheet) directly or through an intermediate transfer body. Thereafter, the toner image is fixed on the recording medium by e.g. a heating and fixing mechanism, thereby forming an image. This heating and fixing mechanism includes a radiant system, a flash system, a two-roller system using a heating roller and a pressing roller, and a free-belt nipping system using the heating roller and a backup free belt. For example, in the two-roller system, a fixing portion is provided with a pair of rollers of a heating roller heated by e.g. a heater and a pressing roller covered with an elastic body. By passing the recording medium transferred with the toner image between the pair of rollers, the toner image not fixed is fixed onto the recording medium. In an image forming apparatus capable of forming a full-color image a four-color toner image of e.g. Y (yellow), M (magenta), C (cyan) and K (black) is duplicated on the recording medium. By passing the recording medium having the four-color toner image between the pair of rollers, the toner image is fixed on the recording medium, thereby forming the full-color image.
The toner employed for fixing such a full-color image is required to have high melting and mixing property when heat is applied, and is preferred to have high “sharp-melting property”. On the other hand, the releasiablity of the toner from the heating roller and others is not sufficient. Particularly, the toner with high sharp melting property is likely to be offset onto the heating roller and others during fixing. In order to obviate such an inconvenience, conventionally, silicon oil was applied on the heating roller to improve the releasibility of the toner, thereby preventing the toner from being offset to the heating roller and others. However, in order to apply the oil such as the silicon oil on the heating roller, a device for supplying the oil must be provided separately. This complicates the image forming apparatus. In addition, it was also problematic that feeling of oil sticking on an exhausted recording medium is inevitable. In order to solve these problems, an apparatus was developed which forms an image using an oil-less toner making external supply of oil unnecessary instead of applying the silicon oil. In this oil-less toner, mould-releasing wax (hereinafter simply referred to as releasing wax) such as polyethylene wax, paraffin wax and silicon wax is added to the toner so that releasibility is given to the toner containing the coloring agent.
As related arts described in Patent Publications, for example, there is a technique for cleaning unwanted matter containing the wax component on the surface of the recording medium after fixing (for example, see JP-A-2002-91205 (Pages 4-5, FIG. 1)). Although not related to the problem for the releasing wax, there are some techniques for subjecting predetermined processing to the recording medium after fixing. For example, some techniques was developed for providing a cooling roller equipped with a large number of slots from which cooling wind is blown to paper (for example, see JP-A-9-281830 (Page 2, FIG. 1)). In order to solve the problem of occurrence of a striped area with low gloss on a recording sheet after the toner image has been fixed, the assignee proposed a technique for providing a plurality of air blowing slots between a fixing device and a transporting roller and blowing air on the toner image forming plane using a fan (see JP-A-2003-21978 (Pages 5-6, FIG. 2)).
Meanwhile, in the case where the oil-less toner described above is employed, generally, the gloss is lost because of the existence of wax. Nevertheless, in recent years, even where such wax is employed, high color-reproducibility and high gloss as in printing has been required as an image quality. Thus, in order to realize the image quality with high gloss, a technique for causing the toner to contain a wax of a material with a high melting property has been investigated. Further, by applying sufficient heat to the toner image in such a manner that thick paper of fine-quality such as “coated paper” is used as a recording medium and the speed (fixing speed) at which the recording medium passes a heating and fixing mechanism is slowed down, the high gloss image was acquired.
In an image forming apparatus provided with such a heating and fixing mechanism, in order to control the posture, traveling direction, exhausting direction, etc. of the recording medium having passed the heating and fixing mechanism, a contact member of e.g. a finger shape, brush shape or roller-shape is used.
However, particularly, in the case where the thick paper such as the coated paper is used, because of its low cooling speed, the toner image on the paper is also maintained at a high temperature. Thus, when the contact member is brought into contact with the recording medium after fixing, a contact mark called “rollermark” is produced on the toner image, resulting in image quality defect. Conventionally, such image quality defect was not a serious problem. However, for example, in the case where the gloss is required as in a photographed image and the image is formed on the entire surface of the recording medium, the image quality defect due to the roller mark becomes a problem.
Particularly, the image quality defect due to the roller mark is likely to occur in the color image with the toners of two or more colors superposed. The image quality defect is also conspicuous when using the toner doped with the releasing wax.